Generally, a mobile communication terminal is a portable device that can provide a voice communication with a person at a distant place. At the beginning, a portable terminal could provide a voice communication only, but it is being developed to a data communication device using letters and symbols as well as video signals.
A code division multiple access (referred to as “CDMA” hereinafter) communication system has been suited for a voice and data communication method using letters and symbols because of relatively smaller data sizes. However, the recent multimedia communication system using video signals requires a communication system that has a faster data transmission speed because of larger data sizes. The system to suit this requirement is the third generation partnership project (3GPP), for example, the wideband code division multiple access (referred to as “WCDMA” hereinafter).
Moreover, the mobile communication terminal can be classified according to the method of accessing a mobile communication network. That is, there can be a portable terminal that provides a voice and data communication by accessing a CDMA mobile communication system, a portable terminal that provides a multimedia communication by accessing a WCDMA mobile communication system, and an MB-MM mobile communication terminal that communicates by selectively accessing a CDMA mobile communication system and a WCDMA mobile communication system.
FIG. 1 shows an illustration of a mobile communication network for providing a WCDMA service in a communication environment in which a CDMA mobile communication network is fundamentally established.
As shown in FIG. 1, the current CDMA mobile communication network 100 is provided in a vast coverage area. However, a WCDMA mobile communication network is provided in sections of the CDMA mobile communication network 100. Therefore, some areas of the CDMA mobile communication network can coincide with the WCDMA mobile communication network, and these areas can be referred to as overlay areas 110. Of course, the WCDMA mobile communication network can exist independently outside a CDMA mobile communication network 100.
Co-present in the overlay areas 110 are both a CDMA mobile communication network 100, which is the second generation mobile communication network, and a WCDMA mobile communication network, which is a third generation mobile communication network. However, a WCDMA mobile communication network is preferably applied in the overlay areas 110, in order to use a WCDMA mobile communication service having a multimedia communication capability. Therefore, a user has to have an MB-MM mobile communication terminal (120-1, 120-2, referred to as “120” hereinafter) that encompasses both a CDMA modem and a WCDMA modem, which are selectively driven, in order to selectively use a WCDMA-type service and a CDMA-type service inside and outside the overlay areas (hereinafter referred to as “WCDMA mobile communication network”) 110. Therefore, intersystem handover between the modems equipped in an MB-MM mobile communication terminal 120 is needed in order to provide an uninterrupted service in case a user moves from a WCDMA mobile communication network 110 to a CDMA mobile communication network while using the MB-MM mobile communication terminal 120.
Generally, a WCDMA modem equipped in an MB-MM mobile communication terminal 120 is always operating while the MB-MM mobile communication terminal 120 is operating (even though the WCDMA modem is within a CDMA mobile communication network 100) and controls the activation and/or termination of a CDMA modem. For example, in case an MB-MM mobile communication terminal 120 moves from a WCDMA mobile communication network 100 to a CDMA mobile communication network 100, a WCDMA modem transmits an activation order (for activating a CDMA modem) to a CDMA modem in order to carry out the handover. On the other hand, a WCDMA modem transmits an termination order (for terminating a CDMA modem) to a CDMA modem in case a conventional MB-MM mobile communication terminal 120 moves from a CDMA mobile communication network 100 to a WCDMA mobile communication network 120. The CDMA modem that received an activation order transmits an activation confirmation signal (e.g., ACK signal) to a WCDMA modem if the modem is properly activated. Moreover, the CDMA modem that received a termination order transmits a termination confirmation signal (e.g., ACK signal) to a WCDMA modem if the modem is properly terminated.
The WCDMA modem can verify a normal activation of the CDMA modem through the activation confirmation signal. Moreover, the WCDMA modem can verify a normal termination of the CDMA modem through the termination confirmation signal. If no negative confirmation signal (e.g. NACK) corresponding to an activation order of the WCDMA modem is received for a predetermined duration, the WCDMA modem can regard that an activation confirmation signal is received from the CDMA modem. This mechanism is identical when the WCDMA modem outputs a termination order.
If a WCDMA modem controls the activation and/or termination of a CDMA modem by a method described above, it is possible that the CDMA modem is not activated and/or terminated normally because the activation order and/or termination order transmitted from the WCDMA modem is lost. Moreover, if a WCDMA modem does not receive an NACK signal corresponding to an activation termination order transmitted to a CDMA modem, and thereby regards that the CDMA modem is normally terminated (although the CDMA modem is not actually terminated), it is possible that the WCDMA modem erroneously retransmits an activation order to a CDMA modem that is already activated for reasons such as handover. Conversely, the same problem is possible for the case of a WCDMA modem transmitting a termination order to a CDMA modem. Moreover, if a CDMA modem is regarded as terminated although the CDMA modem is not terminated, it can activate both the CDMA modem and the WCDMA modem, causing serious drain of battery power in the terminal.